The present invention is related to a communication network comprising a primary station coupled to a plurality of secondary stations via a transmission medium, the secondary stations being arranged for attempting to seize a channel for transmitting information to the primary station.
The present invention also relates to a primary station and a secondary station for use in such a communication network. The invention is also related to a communication method.
A communication network according to the preamble is known from the article xe2x80x9cPerformance of PRMA: A Packet Voice Protocol for Cellular systemsxe2x80x9d by S. Nanda,
D. Goodman and U. Timor in IEEE Transactions on Vehicular Technology, Vol.40, No.3, August 1991.
Such communication systems are used when a plurality of secondary stations have to transmit information to a primary station over a transmission medium being partly in common for a number of secondary stations. In order to prevent mutual inference between two or more secondary stations transmitting information to a primary station, several techniques as FDMA, TDMA, CDMA and combinations thereof have been developed.
In FDMA (Frequency Division Multiple Access) the channels comprise a plurality of frequency bands, each being able to carry one signal from a secondary terminal. By allocating one of the available frequency bands to a secondary station having data to transmit to the primary station, it becomes possible that a plurality of secondary stations transmit information to the primary station without interfering each other.
In TDMA (Time Division Multiple Access) the time axis is divided into a plurality of time slots, in each of which a different secondary station can transmit information to the primary station.
In CDMA (Code Division Multiple Access) to each of the (active) secondary stations a code is assigned. The secondary station uses this code to modulate the data to be transmitted which in general has a much lower transmission rate than the rate which is used for transmitting the code. The primary station can extract the signal transmitted by a particular secondary station by correlating the received signal with the code assigned to said particular station. The received signals from the different secondary stations are orthogonal, because the codes assigned to the different secondary stations are mutually orthogonal.
It is observed that it also possible to use combinations of the above mentioned multiple access methods. An often used combination is a combination of FDMA and TDMA in which a plurality of frequency bands is available for transmitting information from the secondary stations to the primary stations. In each of the frequency bands a frame with a plurality of time slots is used. This particular combination is e.g. used in GSM (Global System for Mobile Communication) and DECT (Digital Enhanced Cordless Telephone).
In large communication networks, the available channels (frequencies, time slots, codes or combinations thereof) is smaller than the number of secondary stations, making it impossible to assign fixedly a channel to a secondary station. To enable the use of the available channels by all secondary stations, a Medium Access (MAC) protocol is used. In the prior art according to the above mentioned article the secondary stations determine which channels are available for seizing in a next frame by listening to the acknowledgment messages transmitted by the primary station after correct reception of a message from a secondary station. In a next frame the secondary stations attempt to seize one of the channels which were not used in the previous frame.
A problem with the prior art access protocol is that the secondary stations need to be able to receive and decode all acknowledgment messages transmitted by the primary station. For high capacity networks the reception and processing of all acknowledge messages in each secondary station is virtually impossible due to the large number of channels to be monitored.
An object of the present invention is to provide a communication network according to the preamble which is able to deal with a large number of channels.
To achieve said object the communication network according to the present invention is characterized in that the primary station is arranged for determining which channels are not used by a secondary station, in that the primary station is arranged for transmitting to the secondary stations an identification of channels which are not used by secondary stations, and in that the secondary stations are arranged for attempting to seize at least one of the channels which are not used by secondary stations.
In this way it is obtained that the secondary stations do not have to receive and process all acknowledge messages, but that it can simply determine the channels which it can try to seize in the next slot by receiving a list of free channels determined by the primary station.
It is observed that it is also conceivable that the primary station transmits to the secondary stations which channels are in use. The secondary station can derive from this information the channels which they can use for transmitting transmit request signals. The attempt to seize a channel can be done by transmitting a special transmit request signal to the primary station, but preferably a first part of the payload data is transmitted to the primary station to seize a channel. The advantage of using payload data to seize a channel is the increased efficiency.
An embodiment of the present invention is characterized in that the primary station is arranged for transmitting to the secondary stations whether a particular attempt to seize a channel was successful, and in that the secondary station having successfully seized said channel, keeps using said channel for transmitting data to the primary station.
In this embodiment of the invention, the primary station does not need to transmit information to the secondary station on which channel it should transmit its payload information. This simplifies the access protocol substantially. It is observed that the signaling to the secondary station that its attempt to seize the channel was successful can simply be done by removing/adding the particular channel from/to the list of unused/used channels, leading to a further decrease of the complexity of the access protocol.
A further embodiment of the invention is characterized in that the primary station is arranged for determining a channel again as available channel as soon the secondary station stops transmitting data over said channel.
This feature simplifies the access protocol further, because no signaling mechanism is required to inform the primary station that a secondary station does not need the channel anymore. The secondary station can simply stop using the channel and the channel will automatically assumed to be free.
A still further embodiment of the invention is characterized in that the secondary station is arranged for attempting to seize a channel with a probability p smaller than one, if a previous attempt to seize a channel was not successful.
In this way it is prevented that in case of a huge amount of attempts of seizing a channel these attempts keep colliding without any channel being seized. By reducing the probability that an attempt of seizing a channel takes place, the number of attempts of seizing a channel that will be successful can be substantially increased.
A still further embodiment of the invention is characterized in that the secondary stations are arranged for randomly selecting one of the available channels to be seized by the secondary station.
According to this embodiment it is ensured that the attempts to seize a channel is evenly distributed over the channels, leading to a maximum probability of a successful seizure of a channel for all secondary stations.
A still further embodiment of the present invention is characterized in that the secondary station is arranged for attempting to seize at least one additional channel if it receives an indication from the primary station that its previous attempt to seize a channel was successful.
This feature enables a secondary station to obtain a transmission capacity being larger than the transmission capacity of one channel. By attempting to seize a new channel after a previous attempt to seize a channel that was successful, the amounts of attempts to seize a channel is distributed in time. This increases the probability of a successful attempt of seizing a channel.